Source code:
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Imports of the form from__future__importfeature are called
. These are special-cased by the Python compiler to allow the use of new Python features in modules containing the future statement before the release in which the feature becomes standard.
While these future statements are given additional special meaning by the Python compiler, they are still executed like any other import statement and the __future__ exists and is handled by the import system the same way any other Python module would be. This design serves three purposes:
To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.
To document when incompatible changes were introduced, and when they will be — or were — made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing __future__ and examining its contents.
To ensure that
run under releases prior to Python 2.1 at least yield runtime exceptions (the import of __future__ will fail, because there was no module of that name prior to 2.1).
Module Contents
No feature description will ever be deleted from __future__. Since its introduction in Python 2.1 the following features have found their way into the language using this mechanism:
feature
optional in
mandatory in
effect
__future__.nested_scopes
2.1.0b1
2.2
: Statically Nested Scopes
__future__.generators
2.2.0a1
2.3
: Simple Generators
__future__.division
2.2.0a2
3.0
: Changing the Division Operator
__future__.absolute_import
2.5.0a1
3.0
: Imports: Multi-Line and Absolute/Relative
__future__.with_statement
2.5.0a1
2.6
: The “with” Statement
__future__.print_function
2.6.0a2
3.0
: Make print a function
__future__.unicode_literals
2.6.0a2
3.0
: Bytes literals in Python 3000
__future__.generator_stop
3.5.0b1
3.7
: StopIteration handling inside generators
__future__.annotations
3.7.0b1
Never
: Postponed evaluation of annotations,
: Deferred evaluation of annotations using descriptors
class__future__._Feature
Each statement in __future__.py is of the form:
FeatureName=_Feature(OptionalRelease,MandatoryRelease,CompilerFlag)where, normally, OptionalRelease is less than MandatoryRelease, and both are 5-tuples of the same form as
:
(PY_MAJOR_VERSION,# the 2 in 2.1.0a3; an intPY_MINOR_VERSION,# the 1; an intPY_MICRO_VERSION,# the 0; an intPY_RELEASE_LEVEL,# "alpha", "beta", "candidate" or "final"; stringPY_RELEASE_SERIAL# the 3; an int)_Feature.getOptionalRelease()
OptionalRelease records the first release in which the feature was accepted.
_Feature.getMandatoryRelease()
In the case of a MandatoryRelease that has not yet occurred, MandatoryRelease predicts the release in which the feature will become part of the language.
Else MandatoryRelease records when the feature became part of the language; in releases at or after that, modules no longer need a future statement to use the feature in question, but may continue to use such imports.
MandatoryRelease may also be None, meaning that a planned feature got dropped or that it is not yet decided.
_Feature.compiler_flag
CompilerFlag is the (bitfield) flag that should be passed in the fourth argument to the built-in function
to enable the feature in dynamically compiled code. This flag is stored in the _Feature.compiler_flag attribute on
instances.
See also
How the compiler treats future imports.
- Back to the __future__The original proposal for the __future__ mechanism.